1. Field of the Invention
This invention relates to a process for producing a silicon carbide sintered body having excellent strength and toughness and high corrosion resistance and being particularly suitable as a contacting part requiring excellent corrosion resistance under severe conditions, such as a skid button to be in contact with iron at high temperatures. The present invention also relates to a novel silicon carbide sintered body having a fine structure.
2. Description of the Prior Art
Because of its various excellent properties such as oxidation resistance, corrosion resistance, heat resistance, thermal shock resistance and high-temperature strength, the silicon carbide sintered body is a suitable material for high-temperature gas turbine parts, automobile engine parts, corrosion-resistant parts and abrasion-resistant parts. However, since silicon carbide is difficult to sinter, it has been the practice to obtain a dense and compact sintered body of silicon carbide by adding various sintering aids. Boron (B)-carbon (C) type additives are examples of the sintering aids. To obtain a dense sintered body, firing at relatively high temperatures is necessary. Furthermore, since the crystalline structure of the sintered body generally consists of isometric grains, there is very little grain boundary phase. Owing to this, therefore, the toughness of the sintered body is low, and a further improvement in corrosion resistance at high temperatures cannot be expected. For example, the corrosion resistance of such a sintered body is still not satisfactory enough for use as a material for skid buttons which come into contact with iron, etc. at high temperatures.
As is well known, by using a silicon carbide sintered body as a material for a skid button, a cooling source for the skid button can be obviated, and at the same time, the occurrence of skid marks is reduced and the thermal efficiency of the inside of a heating furnace can be improved.
In using the skid button, however, a slab heated to the red-hot state comes into contact with the silicon carbide sintered body. At this time, the siliceous component in silicon carbide reacts with iron to form a molten reaction product, and the corrosion of the sintered body easily proceeds.
The use of aluminum-type additives such as alumina (Al.sub.2 O.sub.3) easily gives a dense sintered body. But it is known in this case that since the sintered body has a large volume, the density of the sintered body becomes non-uniform between its inside and outside. Specifically, when an aluminum-type additive is used, it must be added in an amount of at least 3% by weight as aluminum. Thus, the amount of grain boundary components is large and the amount of decomposition thereof is large. The density of the sintered body varies from its inside to its outside, and no excellent corrosion resistance is obtained.